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jema
Downsizer Moderator
Joined: 28 Oct 2004
Posts: 28238
Location: escaped from Swindon
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 Posted: Sun Jul 03, 05 7:03 pm Post subject: |
 
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Halogen Bulb Failure Modes
Halogen bulbs usually fail the same way that ordinary incandescent bulbs do, usually from melting or breakage of a thin spot in an aging filament.
Thin spots can develop in the filaments of halogen bulbs, since the filaments can evaporate unevenly and the halogen cycle does not redeposit evaporated tungsten in a perfect, even manner nor always in the parts of the filament that have evaporated the most.
However, there are additional failure modes.
One failure mode is filament notching or necking. Since the ends of the filament are somewhat cool where the filament is attached to the lead wires, the halogen attacks the filament at these points. The thin spots get hotter, which stops the erosion at these points. However, parts of the filament even closer to the endpoints remain cool and suffer continued erosion. This is not so bad during continuous operation, since the thin spots do not overheat. If this process continues long enough, the thin spots can become weak enough to break from the weight of the filament.
One major problem with the "necked" ends of the filament is the fact that they heat up more rapidly than the rest of the filament when the bulb is turned on. The "necks" can overheat and melt or break during the current surge that occurs when the bulb is turned on. Using a "soft-start" device prevents overheating of the "necks", improving the bulb's ability to survive "necking". Soft-start devices will not greatly extend the life of any halogen bulbs that fail due to more normal filament "thin spots" that run excessively hot.
Some halogen bulbs may usually burn out due to filament end necking, and some others may usually burn out from thin, hot spots forming in the filament due to uneven filament evaporation/recovery. Therefore, some models may have a significantly extended life from "soft-starting" and some other models may not.
It is generally not a good idea to touch halogen bulbs, especially the more compact, hotter-running quartz ones. Organic matter and salts are not good for hot quartz. Organic matter such as grease can carbonize, leaving a dark spot that absorbs radiation from the filament and becomes excessively hot. Salts and alkaline materials (such as ash) can sometimes "leach" into hot quartz, which typically weakens the quartz, since alkali and alkaline earth metal ions are slightly mobile in hot glasses and hot quartz. Contaminants may also cause hot quartz to crystalize, weakening it. Any of these mechanisms can cause the bulb to crack or even violently shatter. If a quartz halogen bulb is touched, it should be cleaned with alcohol to remove any traces of grease. Traces of salt will also be removed if the alcohol has some water in it.
Since the hotter-running quartz halogen bulbs could possibly violently shatter, they should only be operated in suitable fully enclosed fixtures.
Use of Halogen Bulbs with Dimmers
Dimming a halogen bulb, like dimming any other incandescent lamp, greatly slows down the formation of thin spots in the filament due to uneven filament evaporation. However, "necking" or "notching" of the ends of the filament remains a problem. If you dim halogen lamps, you may need "soft-start" devices in order to achieve a major increase in bulb life.
Another problem with dimming of halogen lamps is the fact that the halogen cycle works best with the bulb and filament at or near specific optimum temperatures. If the bulb is dimmed, the halogen may fail to "clean" the inner surface of the bulb. Or, tungsten halide that results may fail to return tungsten to the filament. Halogen bulbs have sometimes been known to do strange and scary things when greatly dimmed.
Halogen bulbs should work normally at voltages as low as 90 percent of what they were designed for. If the bulb is in an enclosure that conserves heat and a "soft-start" device is used, it will probably work well at even lower voltages, such as 80 percent or possibly 70 percent of its rated voltage. However, do not expect a major life extension unless soft-starting is used. Even with soft-starting, do not expect to more than double or possibly triple the life of any halogen bulb already rated to last 2,000 hours or more. Even with soft starting, the life of these bulbs will probably not continue to improve much as voltage is reduced to less than about 90 percent of the bulb's voltage rating.
Dimmers can be used as soft-start devices to extend the life of any particular halogen bulbs that usually fail from "necking" of the ends of the filament. The bulb can be warmed up over a period of a couple of seconds to avoid overheating of the "necked" parts of the filament due to the current surge that occurs if full voltage is applied to a cold filament. Once the bulb survives starting, it is operated at full power or whatever power level optimizes the halogen cycle (usually near full power)
The dimmer may be both "soft-starting" the bulb and operating it at slightly reduced power, a combination that often improves the life of halogen bulbs. Many dimmers cause some reduction in power to the bulb even when they are set to maximum.
(A suggestion from someone who starts expensive medical lamps by turning up a dimmer and reports major success in extending the life of expensive special bulbs from doing this.)
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Interesting stuff from:
https://members.misty.com/don/bulb1.html#hb |
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tawny owl
Joined: 29 Apr 2005
Posts: 563
Location: Hampshire
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